9 illustrates the working of a block of ground by the top-slice caving system.
The subdrift caving system, fig.
The bottom-slice caving system of mining begins at the bottom of a hundred-foot block of ground, a floor being excavated under the whole area, leaving pillars of sufficient size to support the ground above.
While the mine workings are small the overlying rocks support themselves of and the full pressure does not come upon the mine Caving p i llars.
The caving-in of the mine, however, is rarely so complete that avenues of escape are not open.